Broadband integrated digital services are possible in network architectures having fiber in the subscriber loop. Many different network designs have been proposed to handle the broadband service such as BISDN. While passive optical network designs are becoming a network characteristic of choice, the strategy governing communication between the subscriber and the exchange or central office is far from settled. Most proposed strategies involve the use of a single multiplexing technique such as either time-division (TDM) or wavelength-division (WDM) or subcarrier (SCM) multiplexing.
One exemplary network was reported by McGregor et al. in Journal of Lightwave Technology, Vol. 7, No. 11, pp. 1752-8 (1989) involving the use of TDM for both downstream (exchange to subscriber) and upstream (subscriber to exchange) data transmission and WDM to separate the downstream transmission from the upstream transmission. This network utilized bidirectional transmission over a single optical fiber by requiring the presence of 10 .mu.s time guard bands around the data for each transmission direction. The time guard bands prevented upstream and downstream data collisions. A wavelength independent, three-port optical connector at the exchange end of the network facilitated the insertion and removal of data.
Even though the prototype described in the article utilized one fiber for bidirectional transmission, improvements to the prototype included the use of separate fibers for each transmission direction. This improvement offered the dual effects accomodating optical reflections in practical optical distribution plants and eliminating the need for time guard bands. In this dual fiber architecture, McGregor et al. suggested the use of subcarrier multiplexing over the upstream fiber. Unfortunately, the improvements required a duplication of optical fiber installations to support each separate transmission direction. Additionally, the number of splices, connectors, and couplers doubled over the single, bidirectional transmission fiber network. As such the proposed network is incapable of being deployed over existing optical fiber installations without significantly increased costs.